In recent years, a personal authentication technique utilizing iris recognition has been started to be used for entrance/exit management in a restricted building, bank ATMs (Automated Teller Machines), PC login applications, etc.
Japanese Patent Publication for Opposition No. 5-84166 (hereinafter, referred to as Document 1) discloses a basic iris recognition method wherein iris image information obtained at the time of authentication is compared with stored iris image information to identify a person. In this method, the pupil diameter is changed by illuminating an eye, and the pupil diameter is set to a predetermined diameter by controlling the illumination intensity. Thereafter, iris images or feature amounts extracted from the iris images are compared with each other. The diameter of the pupil changes according to the environmental brightness, the state of feeling, or the like. If the pupil diameters are different, mutual comparison of iris images or feature amounts is difficult. Thus, in Document 1, the pupil diameter is adjusted by illumination before comparison. However, a certain time is required for changing the pupil diameter to a predetermined diameter, and accordingly, authentication takes time.
The above problem is solved in Japanese Unexamined PCT National Phase Publication No. 8-504979 (Document 2) and Japanese Unexamined Patent Publication No. 2000-194855 (Document 3) as described below.
Document 2 proposes a feature extraction method which does not depends on the pupil diameter, wherein an iris region is expressed in a polar coordinate system. Specifically, the iris region is expressed in a polar coordinate system where the center of the pupil/iris border approximated to a circle is the origin point of the system, and the radial coordinate is expressed by the proportion of the distance between the pupil/iris border and the iris/sclera border. With such a method, even when the pupil diameter measured at the time of registration is different from that measured at the time of authentication, features in a certain expression are extracted, and the amounts of the extracted features are compared with each other. Thus, it is not necessary to control the pupil diameter at the time of authentication, and accordingly, the authentication time is shortened as compared with the method of Document 1.
In Document 3, at the time of registration, a plurality of iris images having different pupil diameters are captured by using different illumination intensities, the feature amounts extracted from the plurality of iris images are registered, and a feature amount extracted from an iris image captured at the time of authentication is compared with the plurality of registered feature amounts. Even if the pupil diameter of an iris image captured at the time of authentication has an arbitrary value, the pupil diameter can be compared with registered data which has a substantially-equal pupil diameter because a plurality of iris images having different pupil diameters have been captured at the time of registration. Thus, it is not necessary to control the pupil diameter at the time of authentication, and accordingly, the authentication time is shortened as compared with the method of Document 1.